Currently, for a switch for the igniter, which is an automotive ignition device, there have been widely used insulated gate bipolar transistors (IGBT), and in particular, in recent years, IGBTs for igniters, each incorporating a protection function therein, have been increasing. The IGBT incorporating the protection function has been disclosed in Japanese Patent Laid-Open No. 9-280147/1997.
In the Japanese Patent Laid-Open No. 9-280147/1997, there has been disclosed an inhibition method for current vibration of the IGBT incorporating a current limiting function. FIG. 2 shows the IGBT and its current limiting circuit disclosed in the Japanese Patent Laid-Open No. 9-280147/1997. In FIG. 2, reference numeral 201 designates a battery; 202, an ignition coil; 203, IGBT; 204, a spark plug; 205, a resistor; 206, a high voltage constant current element; 207, a Zener diode; 208, a diode; 209, 210, a resistor; 211, a capacitor; 212, a transistor; 213, a resistor; and 214, a drive circuit.
The description will be made of an operation of this circuit. In the beginning, a state in which the IGBT203 is off will be considered. In this state, no signal is outputted from the drive circuit 214, but a gate of an IGBT 203 is at 0 V. On the other hand, since the IGBT203 is off, there is no current flowing through a so-called main circuit reaching from the battery 201 to the IGBT204 through the ignition coil 202, but a battery voltage is applied between collector and emitter of the IGBT204.
Next, the description will be made of an on-state of the IGBT. When an on-signal from the drive circuit 214 is inputted into a gate of the IGBT204, the IGBT203 is turned on, and current starts to flow through a main circuit. Next, when the IGBT is turned off again, as high voltage as hundreds volt is developed across a primary terminal of the ignition coil 202 because the current of the main circuit is abruptly cut off. This voltage is raised to tens of thousands volts and conducted on a secondary terminal side of the ignition coil to discharge the spark plug.
The circuit is described to have two types of protection circuits. One of them is an overvoltage protection circuit. In a case where when the IGBT shifts from an on-state to an off-state, discharge cannot be performed because of some trouble or other with the spark plug, voltage at the primary terminal of the ignition coil 202 continues to be raised. For this reason, there is, between collector and gate of the IGBT, provided the Zener diode 207 which limits the voltage when the predetermined voltage is exceeded.
The other type is a current limiting circuit. The maximum value of current flowing through the IGBT becomes higher in proportion to a time period during which the IGBT is on by the drive circuit, and when this time period becomes long because of some trouble or other, there may be cases where a large current flows through the IGBT. Thus, there is provided a function which when the current through the IGBT exceeds a predetermined current value, voltage to be developed across the resistor 205 causes the transistor 212 to turn on for reducing gate voltage of the IGBT, and limits the current flowing through the IGBT.
Although this current limiting circuit had a problem that current vibration is prone to occur, in the Japanese Patent Laid-Open No. 9-280147/1997, by adding a circuit for supplying low current from the collector to the gate during the current limiting operation, that is, the high voltage constant current element 206 and the resistor 209, this problem has been solved.
In the Japanese Patent Laid-Open No. 9-280147/1997, however, since the structure is arranged such that current is supplied from the collector to the gate in order to inhibit the vibration, there is a problem that leakage current from the collector to the gate is high. This leakage current is current that continues to flow from the battery in a state in which the IGBT is off, or current that flows from the collector to the gate when high voltage is applied, such as during discharge from the spark plug.
In order to reduce the leakage current, an amount of current supply from the collector to the gate can be reduced, and on the other hand, in order to inhibit the current vibration, it is necessary to supply current to some extent from the collector to the gate. The current to some extent is current enough to, when vibration of gate voltage which causes the current vibration is going to occur, inhibit this vibration, and this current is determined by impedance of the drive circuit, gate-emitter capacity of IGBT, or the like. In the case of a particularly large IGBT chip, since the gate-emitter capacity also becomes large, there arises the need for supplying a large current responsive thereto from the collector, and the leakage current is also increased. When this leakage current is increased, loss in the off-state is increased, resulting in a problem that electricity stored in the battery is consumed. Also, during ignition to the spark plug, arc energy is reduced by an amount corresponding to the loss due to the leakage current, which will cause automotive combustion efficiency to be deteriorated among others. Further, it will become impossible to check collector-emitter leakage current, which is to be performed during selection of an initial failure of the IGBT, and it has also a problem of reliability degradation.
It is an object of the present invention to solve the above-described problems and to provide a highly reliable ignition circuit for an internal combustion engine with little loss.